s390/boot: rename HEAP_SIZE due to name collision

Change the conflicting macro name in preparation for zlib_inflate
hardware support.

Link: http://lkml.kernel.org/r/20200103223334.20669-3-zaslonko@linux.ibm.com
Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>